Index guided VCSEL and method of fabrication

ABSTRACT

A reliable high frequency VCSEL includes a lower distributed Bragg reflector (DBR), an active region, and an upper DBR. A cylindrical volume is etched from the upper DBR to define a mesa with a lower surface of the cylindrical volume forming an angle greater than ninety degrees with the side wall of the mesa. An isolation trench is etched in the lower surface of the cylindrical volume concentric with the mesa and extending through the active region. A portion of the side wall of the mesa and the lower surface of the cylindrical volume are proton implanted. The upper DBR is planarized using low-k dielectric materials and n and p electrical contacts are coupled to opposite sides of the active region for supplying operating current thereto.

FIELD OF THE INVENTION

This invention relates to vertical cavity surface emitting lasers and,more particularly, to semiconductor lasers which operate reliably athigh frequencies.

BACKGROUND OF THE INVENTION

Vertical cavity surface emitting lasers (hereinafter referred to as“VCSELs”) have become the dominant light source for optical transmittersused in short-reach local area networks and storage area networkapplications, in which a multi-mode optical fiber is used for datatransmission. VCSELs are low cost micro-cavity devices with high speed,low drive current and low power dissipation, with desirable beamproperties that significantly simplify their optical packaging andtesting. In order to extend the application of VCSELs to higher speedapplications, the VCSEL must be capable of operating reliably atfrequencies of up to 10 GHz.

Commercial oxide confined VCSELs have been widely deployed in the field.However, due to intrinsic mechanical stress introduced by the oxidationin the VCSEL fabrication, oxide confined VCSELs are not as reliable as,for example, proton (or ion) implanted VCSELs with higher random failurerates. Prior art VCSELs which include an oxide confinement may operateat 10 GHz, but they suffer from poor reliability. Prior art ionimplanted VCSELs typically operate at about 1 GHz, but-are more reliablethan VCSELs with oxide confinement. Although certain stress reliefmethods may be introduced to reduce the random failure rate, theoxidation process is too sensitive to the temperature, materialscomposition, and gas pressure during device fabrication and, therefore,the oxide confinement process is not a consistent manufacturing processfor VCSELs.

Ion implanted VCSELs are relatively more reliable. However, ionimplanted devices do not perform well at higher speeds and, therefore,their applications are limited to data rates around 1 Gbps. The speed ofan ion implanted VCSEL is limited by several factors. One factor is thelack of a good index guiding for the optical mode. Another factor isfrom a size limitation due to a deep implant where the typical implantdepth may be more than three microns. Further, the implant has adistribution with a large straggle and a large standard deviation. Witha large implant distribution and the poor current confinement of aheavily doped mirror, the size is typically more than 20 microns whereinthe speed is limited to less than 2 GHz.

Therefore, there is a need to develop a reliable high performance VCSELfor high speed optical communications.

Accordingly, it is an object of the present invention to provide new andimproved VCSELs that operate reliably at high frequencies.

It is another object of the present invention to provide new andimproved VCSELs with reduced current leakage and device capacitance.

SUMMARY OF THE INVENTION

Briefly, to achieve the desired objects of the instant invention inaccordance with a preferred embodiment thereof, a reliable highfrequency vertical cavity surface emitting laser (VCSEL) is provided.The VCSEL includes a lower distributed Bragg reflector, an active regionpositioned on the lower distributed Bragg reflector, and an upperdistributed Bragg reflector positioned on the active region. Acylindrical volume is etched from the upper distributed Bragg reflectorso as to define a mesa with a substantially vertical side wallconcentrically surrounded by the cylindrical volume. An isolation trenchis etched in a lower surface of the cylindrical volume concentric withthe mesa. An implant region is formed in the cylindrical volume,including a portion of the side wall of the mesa and a portion of theupper distributed Bragg reflector below the lower surface of thecylindrical volume. The cylindrical volume is filled with a dielectricor insulating material to planarize the VCSEL for further isolation andpassivation. Electrical contacts are coupled to opposite sides of theactive region for supplying operating current thereto.

The desired objects of the instant invention are further achievedthrough a novel method of fabricating a high frequency vertical cavitysurface emitting laser. The method includes providing a lowerdistributed Bragg reflector on a substrate, an active region on thelower distributed Bragg reflector, and an upper distributed Braggreflector on the active region. The method also includes etching acylindrical volume from the upper distributed Bragg reflector to definea mesa with a substantially vertical side wall, the cylindrical volumeextending into the upper distributed Bragg reflector to a lower surfaceadjacent the active region and etching an isolation trench in the lowersurface of the cylindrical volume concentric with the mesa and extendingthrough the active region. The method further includes a step ofimplanting a portion of the side wall of the mesa and the lower surfaceof the cylindrical volume and planarizing the upper distributed Braggreflector. Finally, coupling n and p electrical contacts are coupled toopposite sides of the active region for supplying operating currentthereto.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and further and more specific objects and advantages ofthe instant invention will become readily apparent to those skilled inthe art from the following detailed description of a preferredembodiment thereof taken in conjunction with the following drawings:

FIG. 1 is a simplified sectional view of a high speed vertical cavitysurface emitting laser in accordance with the present invention; and

FIG. 2 is a simplified top plan view of the high speed vertical cavitysurface emitting laser illustrated in FIG. 1.

DETAILED DESCRIPTION OF THE DRAWINGS

Turning now to FIG. 1, a simplified sectional view of a vertical cavitysurface emitting laser (VCSEL) 10 is illustrated. Also, FIG. 2illustrates a top plan view of complete VCSEL 10, which may be referredto in addition to FIG. 1 throughout this discussion. VCSEL 10 includes asubstrate 12 which may be, for example, some convenient single crystalsemiconductor material, such as gallium arsenide (GaAs) or the like. Asis well known in the art, in most instances a thin buffer layer 14 ofthe same material is provided to ensure a smooth crystalline surface forfurther growth processes. Layer 14 is considered part of the substratein this disclosure. A lower mirror stack or distributed Bragg reflector(DBR) 16 is grown on the upper surface of substrate 12. An active region18 which, as is known in the art, may include cladding regions or thelike (not shown) on opposite sides, is grown on the upper surface of DBR16. An upper mirror stack or distributed Bragg reflector (DBR) 20 isgrown on the upper surface of active region 18. As is understood bythose skilled in the art, the various layers and/or regions describedabove are generally grown epitaxially in a well known continuousprocedure. Also, the chosen deposition technique is not meant to limitthe scope of the invention. For example, in this embodiment, the basicstructure of VCSEL 10 is formed using metallorganic chemical vapordeposition (MOCVD). However, it will be understood that VCSEL 10 may beformed using chemical vapor deposition, sputtering, molecular beamepitaxy, or combinations thereof. Further, although a single VCSEL isillustrated, generally, a plurality of VCSELs are deposited or formed inblanket layers over an entire wafer so that a large number of VCSELs arefabricated simultaneously.

In this embodiment, substrate 12 includes gallium arsenide (GaAs).However, the choice of substrate material and the material included inactive region 18 generally depends on a desired wavelength of operationwhich in this embodiment is between approximately 0.7 μm to 1.0 μm. Itwill also be understood that the wavelength range from 0.7 μm to 1.0 μmis typically used in optical communication applications, such as fiberoptical networks. However, other wavelength ranges may be suitable for agiven application.

As is understood in the art, DBRs 16 and 20 include a stack of alternatelayers of materials wherein each adjacent layer has a different index ofrefraction. For example, DBRs 16 and 20 in this embodiment includealternate layers of semiconductor material, such as alternate layers ofan alloy of AlGaAs, with different proportions of material to change theindex of refraction, or alternate layers of aluminum arsenide (AlAs) andgallium arsenide (GaAs). It will be understood that DBRs 16 and 20 mayinclude other suitable reflective materials that are stacked alternatelybetween a high and a low index of refraction. Further, in the preferredembodiment, each layer in DBRs 16 and 20 have thicknesses approximatelyequal to one quarter of the wavelength of operation to provide a desiredreflective property. Also, while lower DBR 16 is illustrated as beingdoped for n-type conductivity and upper DBR 20 is illustrated as beingdoped for p-type conductivity, it will be understood that theconductivities could be changed and the present formation is only forpurposes of explanation.

Active region 18 may include from one to a plurality of quantumstructure layers with a band gap wavelength wherein each quantumstructure layer emits light at the wavelength of operation. For example,active region 18 may include layers of aluminum gallium arsenide(AlGaAs), gallium arsenide (GaAS), or indium gallium arsenide (InGaAs).It will be understood that active region 18 may include quantum wells orother device structures with suitable light emission properties, such asquantum dots or similar device structures. The quantum structure layers,quantum wells, quantum dots, etc. are spaced within active region 18 ina well known manner to provide the desired light generation.

Once the basic structure, including lower DBR 16, active region 18, andupper DBR 20 is completed, an etching process is performed in aring-shaped area to remove a cylindrical volume 24 from upper DBR 20 todefine a mesa 25 in upper DBR 20. The etching process continues throughDBR 20 to within a few mirror pairs of active region 18. Further, theetching process is performed so that more mirror pairs remain near thebase of mesa 25 and less mirror pairs remain as the lateral distancefrom mesa 25 (within volume 24) increases. That is, the angle betweenthe vertical side of mesa 25 and the surrounding upper surface of DBR 20is greater than ninety degrees. This ‘rounding’ of the mesa cornerreduces the stress effect within the crystalline material and improvesreliability. Also, mesa 25 is formed to provide an optical and currentconfinement region.

Once volume 24 is etched as described above, a second etch step isperformed within concentric cylindrical volume 24 to form an isolationtrench 28, spaced from mesa 25, that extends through the remainingmirror pairs in upper DBR 20, active region 18, and into lower DBR 16.Isolation trench 28 (and volume 24) extends concentrically around mesa25 and is included to reduce current leakage and device capacitance. Thetrench etching step can be performed before or after an implant step,which is described in detail below, depending upon the convenience andcontinuity of the various steps. For example, a first etch followedimmediately by a second etch may be more convenient than interspersingan implant step between.

With at least the mesa defining etching process completed, a protonimplant process is performed on the side wall of mesa 25 and theremaining DBR mirror pairs immediately under volume 24 to form implantarea 30. Generally, the mirror pairs remaining adjacent mesa 25 aresufficient so that proton implant area 30 adjacent mesa 25 is close butdoes not extend into active region 18. However, as the lower surface ofcylindrical volume 24 extends laterally a greater distance from mesa 25,e.g. adjacent isolation trench 28, implant area 30 gradually begins toextend into active region 18. In a specific embodiment, the implant isfrom one to four DBR pairs of layers, or approximately 0.1 to 0.5 um.This was achieved using an implant energy in a range of approximately 30to 70 KV with proton implant.

The etching of cylindrical volume 24, including the side wall of mesa25, causes crystalline damage with dangling bonds and defects at theetch surface which in turn causes unwanted carrier trapping andnon-radiative carrier recombination. However, implant region 30 has ahigh resistance to electrical current flow so that electrical currentwill not flow in the implanted area and, therefore, non-radiativecarrier recombination cannot occur in the damaged crystalline etchedportions. Also, because implant region is on the surface and relativelythin, deep implants are not required and the entire implant process canbe very accurately controlled. Further, since the implant is adjacentthe surface (very shallow) a less complicated proton implant can beused. Here it should be noted that most prior art implants in VCSELswere made through most or even the entire upper mirror stack, therebyrequiring the lighter ion implants. Implant region 30 is provided tostop or prevent current/carrier recombination activated defectpropagation from the etched side wall of mesa 25 into active area 18 tosubstantially improve the reliability of VCSEL 10. Thus, a combinationof etching and implant procedures are used to fabricate VCSEL 10 withall the advantages of both processes and none or few of thedisadvantages.

VCSEL 10 is then planarized using benzocyclobutene(BCB) dielectric(Cyclotene™ from Dow) or some convenient polyimide materials 32 toprovide for better metallization coverage and to reduce devicecapacitance for high speed operation. In addition, BCB is a low-kdielectric material and further helps reduce the VCSEL parasiticcapacitance. A p-contact metal ring 34 is concentrically deposited onthe upper surface of mesa 25 and an n-contact metal layer 35 is appliedto the rear surface (lower surface in FIG. 1) of substrate 12. Aninsulating and passivating coating 38 of some convenientsilicon-oxide-nitride silicon nitride, or the like is applied to theupper surfaces of VCSEL 10. An opening is provided in coating 38 atleast over a portion of p-contact metal ring 34. A top bond-pad metallayer 40 is deposited in contact with the exposed portion of p-contactmetal ring 34 and extending over a convenient portion of coating 38.

Thus, a reliable high performance VCSEL for high speed opticalcommunications is disclosed. The new and improved VCSELs are constructedto operate reliably at high frequencies and with reduced current leakageand device capacitance. Basically, the new and improved VCSELs arefabricated using a convenient mixture of etching and shallow implantingto provide a device having all of the advantages of both processes whileeliminating substantially all of the disadvantages.

While the steps of the fabrication method have been described, and willbe claimed, in a specific order, it will be clear to those skilled inthe art that various steps and procedures may be performed in differentorders. It is intended, therefore, that the specific order described orclaimed for the various fabrication steps does not in any way limit theinvention and any variations in order that still come within the scopeof the invention are intended to be covered in the claims.

Various changes and modifications to the embodiments herein chosen forpurposes of illustration will readily occur to those skilled in the art.To the extent that such modifications and variations do not depart fromthe spirit of the invention, they are intended to be included within thescope thereof which is assessed only by a fair interpretation of thefollowing claims.

1. A vertical cavity surface emitting laser comprising: a lowerdistributed Bragg reflector; an active region positioned on the lowerdistributed Bragg reflector; an upper distributed Bragg reflectorpositioned on the active region; a cylindrical volume removed from theupper distributed Bragg reflector defining a mesa with a substantiallyvertical side wall concentrically surrounded by the cylindrical volume,an isolation trench formed in a lower surface of the cylindrical volumeconcentric with the mesa; an implant region including a portion of theside wall of the mesa and a portion of the upper distributed Braggreflector below the lower surface of the cylindrical volume; aplanarizing material filling the cylindrical volume; and n and pelectrical contacts coupled to opposite sides of the active region forsupplying operating current thereto; wherein the lower surface of thecylindrical volume is formed so that more mirror pairs of the upperdistributed Bragg reflector remain adjacent the mesa and less mirrorpairs remain as the lateral distance from the mesa increases.
 2. Avertical cavity surface emitting laser as claimed in claim 1 wherein thelower surface of the cylindrical volume forms an angle greater thanninety degrees with the side wall of the mesa.
 3. A vertical cavitysurface emitting laser as claimed in claim 1, wherein the implant regionin the lower surface of the cylindrical volume extends at least into theactive region adjacent the isolation trench.
 4. A vertical cavitysurface emitting laser as claimed in claim 1 wherein the planarizingmaterial filling the cylindrical volume includes a low-k dielectricmaterial.
 5. A vertical cavity surface emitting laser as claimed inclaim 1 wherein the implant region includes as least some of thecylindrical volume surface.
 6. A vertical cavity surface emitting laseras claimed in claim 1 wherein the implant region includes protonimplants.
 7. A vertical cavity surface emitting laser as claimed inclaim 1, wherein implant region in the lower surface of the cylindricalvolume extends into the lower distributed Bragg reflector.
 8. A verticalcavity surface emitting laser comprising: a lower distributed Braggreflector including a plurality of pairs of mirror elements; an activeregion positioned on the lower distributed Bragg reflector; an upperdistributed Bragg reflector including a plurality of pairs of mirrorelements positioned on the active region; a cylindrical volume removedfrom the upper distributed Bragg reflector defining a mesa with asubstantially vertical side wall concentrically surrounded by thecylindrical volume, an isolation trench formed in a lower surface of thecylindrical volume concentric with the mesa, the lower surface of thecylindrical volume, being formed so that more mirror pairs of the upperdistributed Bragg reflector remain adjacent the mesa and less mirrorpairs remain as the lateral distance from the mesa increases whereby thelower surface of the cylindrical volume forms an angle greater thanninety degrees with the side wall of the mesa; an implant regionadjacent a surface of the cylindrical volume including the side wall ofthe mesa and the upper distributed Bragg reflector defining the lowersurface of the cylindrical volume; a planarizing material filling thecylindrical volume; and n and p electrical contacts coupled to oppositesides of the active region for supplying operating current thereto.
 9. Amethod of fabricating a high frequency vertical cavity surface emittinglaser comprising the steps of: a) providing a lower distributed Braggreflector on a substrate, an active region on the lower distributedBragg reflector, and an upper distributed Bragg reflector on the activeregion; b) etching a cylindrical volume from the upper distributed Braggreflector to define a mesa with a substantially vertical side wall, thecylindrical volume extending into the upper distributed Bragg reflectorto a lower surface adjacent the active region; c) etching an isolationtrench in the lower surface of the cylindrical volume concentric withthe mesa and extending through the active region; d) implanting aportion of the side wall of the mesa and the lower surface of thecylindrical volume; and e) planarizing the upper distributed Braggreflector and coupling n and p electrical contacts to opposite sides ofthe active region for supplying operating current thereto; wherein stepb) includes etching the cylindrical volume so that the lower surface ofthe cylindrical volume forms an angle greater than ninety degrees withthe side wall of the mesa, and etching the cylindrical volume so thatmore mirror pairs of the upper distributed Bragg reflector remainadjacent the mesa and less mirror pairs remain as the lateral distancefrom the mesa increases.
 10. The method of claim 9, wherein step d)includes proton implanting the side wall of the mesa and the lowersurface of the cylindrical volume.
 11. The method of claim 10, whereinstep d) includes implanting the lower surface of the cylindrical volumeso that the implant extends at least into the active region adjacent theisolation trench.
 12. The method of claim 10, wherein step d) includesimplanting the lower surface of the cylindrical volume so that theimplant extends into the lower distributed Bragg reflector adjacent theisolation trench.
 13. The method of claim 9, wherein step a) includesepitaxially growing the lower distributed Bragg reflector on thesubstrate, epitaxially growing the active region on the lowerdistributed Bragg reflector, and epitaxially growing the upperdistributed Bragg reflector on the active region.
 14. The method ofclaim 9, wherein step e) includes filling the cylindrical volume withone of benzocyclobutene (BCB) dielectric and a polyimide material.
 15. Amethod of fabricating a high frequency vertical cavity surface emittinglaser comprising the steps of: epitaxially growing a lower distributedBragg reflector on a substrate, epitaxially growing an active region onthe lower distributed Bragg reflector, and epitaxially growing an upperdistributed Bragg reflector on the active region; etching a cylindricalvolume from the upper distributed Bragg reflector to define a mesa withsubstantially vertical side wall, the upper distributed Bragg reflectorbeing etched so that a lower surface of the cylindrical volume forms anangle greater than ninety degrees with the side wall of the mesa, andfurther etching the cylindrical volume so that more mirror pairs of theupper distributed Bragg reflector remain adjacent the mesa and lessmirror pairs remain as the lateral distance from the mesa increases;etching an isolation trench in the lower surface of the cylindricalvolume concentric with the mesa and extending through the active region;proton implanting a portion of the side wall of the mesa and the lowersurface of the cylindrical volume; and planarizing the upper distributedBragg reflector and coupling n and p electrical contacts to oppositesides of the active region for supplying operating current thereto. 16.The method of claim 15 wherein the step of proton implanting the lowersurface of the cylindrical volume includes implanting the lower surfaceof the cylindrical volume so that the implant extends at least into theactive region adjacent the isolation trench.
 17. The method of claim 15wherein the step of planarizing includes filling the cylindrical volumewith one of BCB dielectric and a polyamide.
 18. A vertical cavitysurface emitting laser comprising: a lower distributed Bragg reflectorincluding a plurality of pairs of mirror elements; an active regionpositioned on the lower distributed Bragg reflector; an upperdistributed Bragg reflector including a plurality of pairs of mirrorelements positioned on the active region; a cylindrical volume removedfrom the upper distributed Bragg reflector defining a mesa with asubstantially vertical side wall concentrically surrounded by thecylindrical volume, the lower surface of the cylindrical volume beingformed so that more mirror pairs of the upper distributed Braggreflector remain adjacent the mesa and less mirror pairs remain as thelateral distance from the mesa increases whereby the lower surface ofthe cylindrical volume forms an angle greater than ninety degrees withthe side wall of the mesa; an implant region adjacent a surface of thecylindrical volume including the side wall of the mesa and the upperdistributed Bragg reflector defining the lower surface of thecylindrical volume; a planarizing material filling the cylindricalvolume; and n and p electrical contacts coupled to opposite sides of theactive region for supplying operating current thereto.
 19. A verticalcavity surface emitting laser as claimed in claim 18, wherein theimplant region in the lower surface of the cylindrical volume extends atleast into the active region.
 20. A vertical cavity surface emittinglaser as claimed in claim 18, wherein implant region in the lowersurface of the cylindrical volume extends into the lower distributedBragg reflector.